The present invention relates to an epoxy resin composition useful in the making of cast insulators employed in electric machines.
An epoxy resin combined with an acid anhydride cures to provide a product that has superior electrical, mechanical and chemical properties and which is extensively used as an epoxy resin cast insulator in electric machines including those employed in power transmission and distribution. In order to improve the productivity of epoxy resin cast insulators using a smaller number of molds, a method commonly referred to as the superatmospheric gelling process which is capable of reducing the release time is currently employed. In this method, an epoxy resin blend of interest held within a cold pressurized tank is injected into a heated mold through a pipeline and a die head, while the mold is pressurized to compensate for any contraction of the resin being cured, thereby producing the desired casting within a shortened period of curing. The epoxy resin blend employed in this method must have a low viscosity and a long pot life within the cold pressurized tank, and must be capable of curing rapidly within the heated mold.
Those epoxy resins which exhibit low viscosities at low temperatures have low molecular weights and, hence, they exhibit an extremely high degree of shrinkage during curing and are highly likely to give cured products with sink marks and cracks. In addition, epoxy resins that are highly reactive at elevated temperatures will also exhibit comparatively high reactivity at low temperatures and suffer from a shorter pot life.
Common practice for dealing with these problems is to employ the superatmospheric gelling method with a view to preventing the occurrence of sink marks and cracks during the curing process and to use a latent accelerator for the purpose of extending the pot life of the resin blend.
A problem arises, however, from the fact that epoxy resins of low molecular weights that will exhibit low viscosities at low temperatures are less resistant to thermal shock than the solid or high-viscosity epoxy resins which are commonly employed in ordinary casting methods other than the superatmospheric gelling process. Conventionally, the thermal shock resistance of low-viscosity epoxy resins is improved by addition of plasticity providing agents, such as high-molecular weight oligomers that have molecular weights within the range of from about 500 to 5,000 and which are comprised of polyester, polyether, polybutadiene or the like in the backbone chain. However, as the addition of these oligomers is increased, the viscosity of the epoxy resin is increased significantly while its heat resistance is considerably reduced. On the other hand, if the addition of such oligomers is insufficient, there is little possibility of improvement in the resistance of the product against thermal shock. Plasticity providing agents such as those having polyamide in the backbone chain have the advantage that they will not substantially increase the viscosity of the resin blend but then, the resin blend incorporating such plasticity providing agent is highly reactive and has a shorter pot life.
In the superatmospheric gelling method, an epoxy resin blend having a low viscosity at low temperature is injected into a mold that is heated to a temperature higher than that of the resin blend. Within the mold, the viscosity of the resin blend is reduced temporarily to cause precipitation of the filler, giving rise to such problems as surface defects (e.g. flow marks) on the cured product and unevenness in the properties of the final product. These problems are particularly noticeable when the filler is an alumina powder having a comparatively high specific gravity.
As mentioned before, another disadvantage of the low-viscosity epoxy resin which is suitable for use in the superatmospheric gelling method is that it has poor resistance to heat shock and that its heat resistance has to be sacrificed in order to improve its heat shock resisting properties.